Such a method for transmitting data elements, such a transmitter and receiver, and such a transmission system are already known in the art, e.g. from the specifications of the ANSI (American National Standards Institute, Inc.) Standard on ADSL, the approved version of which has the reference T1E1.413-1995 and title “Network and Customer Installation Interfaces, Asymmetric Digital Subscriber Line (ADSL) Metallic Interface”. Therein, data elements are modulated on a set of carriers. In case of discrete multi tone (DMT) modulation, these carriers have equidistant frequencies. As is indicated in paragraphs 6.9.1.2 and 7.9.1.2 on pages 46 and 58 of the above cited standard, published in 1995, one of the carriers is reserved as a pilot carrier. This pilot carrier is used for synchronisation between transmitter and receiver and is modulated by a constant signal. In a vector plane, wherein the modulation constellation is represented by a collection of points, the pilot carrier is thus represented by a single point. On the transmission medium, e.g. on a telephone line interconnecting the ADSL transmitter and ADSL receiver in the known system, the pilot carrier thus represents a sine or cosine which does not change in phase, amplitude or frequency in time (in case a guard band or cyclic prefix is added whose length does not contain an integer number of periods of the pilot tone, the pilot tone might be discontinu at the edges of the DMT symbol).
A well-known source of narrowbanded or single frequency disturbances is a radio amateur or an AM radio station, which broadcasts radio signals at frequencies close to carrier frequencies. Forward error correction techniques, well-known in the art, can reduce the effect of such disturbances on data carried by the affected carriers. An alternative way to protect data against such interferers, proposed by Peter S. Chow et al. in the article “A multicarrier E1-HDSL Transceiver System with Coded Modulation” from the authors Peter S. Chow, Noafal Al-Dhahir, John M. Cioffi and John A. C. Bingham published in issue N° 3 May/June 1993 of the Journal of European Transactions on Telecommunications and Related Technologies (ETT), pages 257–266, is bitswapping: bit and energy allocations are updated so that the affected carriers carry less data bits then before. This technique requires an additional communication between transmitter and receiver.
Although data transmitted over the telephone line from the transmitter to the receiver may be protected by one of the above mentioned techniques, the presence of noise or an interferer, for instance a radio amateur signal, with a frequency in the vicinity of the frequency of the pilot carrier, may still cause an offset between the received point representing the pilot carrier in the above defined vector plane and the expected point. If this offset in the vector plane is not sufficiently random, it biases the synchronisation mechanism, resulting in a performance degradation. This is e.g. the case if the instantaneous phase of the interferer is very slowly varying in time with respect to the duration of the DMT symbol or if this interferer is constant.